We analyzed the transitional change of telomere length and telomerase activity during differentiation of human peripheralblood (PB) CD34+cells to an erythroid lineage. The following findings were obtained from this projects : 1) We devised a nylon-fiber syringe system to deplete CD14+cells in and found an inverse correlation between the frequency of CD14+cells the intial cell population and the purity of CD34+cells in the final preparation. Using this nylon-fiber syringe system, a method for large scale purification of human PB CD34+cells has been developed. 2) The development of method of lineage specific expansion of PB CD34+cells to an erythroid lineage revealed that HTK,a receptor for tyrosin kinase, express predominantly on erythroid progenitor cells, erythropoietin dependent signal transduction by JAK-STAT pathway in human primary cultured erythroid progenitor cells, and that actin filament is responsible for enucleation of erythroblasts. Telomere length continuosly decrease during erythroid differentiation, however, telomerase activity transiently increased after three days of culture of purified CD34+cells, which was quite different manner compared to that seen in neoplastic cells or cell lines. The mechanism of transient increase of telomerase activity in normal progenitor cells are now under investigation.